A&A Supplement series, Vol. 121, January 1997, 95 - 112
Received February 28; accepted May 21, 1996
E. Carretta
- R.G. Gratton
Send offprint request: E. Carretta. Table 9 also available in electronic form at the
CDS via anonymous ftp 130.79.128.5 or via
http://cdsweb.u-strabg.fr/Abstract.html
Osservatorio Astronomico di Bologna, Via Zamboni 33,
I-40126 Bologna, Italy
Osservatorio Astronomico di Padova, Vicolo
dell'Osservatorio 5, I-35122 Padova, Italy
Dipartimento d'Astronomia, Università di Padova,
Vicolo dell'Osservatorio 5, I-35122 Padova, Italy
We have obtained high resolution, high signal-to-noise ratio CCD echelle
spectra of 10 bright red giants in 3 globular clusters (47 Tuc, NGC 6752 and
NGC 6397) roughly spanning the whole range of metallicities of the galactic
globular cluster system. The analysis of this newly acquired material reveals
no significant evidence of star-to-star variation of the [Fe/H] ratio in these
three clusters.
Moreover, a large set of high quality literature data (equivalent widths
from high
dispersion CCD spectra) was re-analyzed in an homogeneous and
self-consistent way to integrate our observations and derive new metal
abundances for more than 160 bright red giants in 24 globular clusters (i.e.
about 16
of the known population of galactic globulars).
This set was then used to define a new metallicity scale for globular clusters
which is the result of high quality, direct spectroscopic data, of new and
updated model atmospheres from the grid of Kurucz (1992), and of a careful
fine abundance analysis; this last, in turn, is based on a common set of both
atomic and atmospheric parameters for all the stars examined.
Given the very high degree of internal homogeneity, our new scale
supersedes the offsets and discrepancies existing in previous attempts to
obtain a metallicity scale.
The internal uncertainty in [Fe/H] is very small: 0.06 dex (24
clusters) on average, and can be interpreted also as the mean precision of the cluster
ranking.
Compared to our system, metallicities on the widely used
Zinn and West's scale
are about 0.10 dex higher for [Fe/H]>-1, 0.23 dex lower for -1<[Fe/H]<-1.9
and 0.11 dex too high for [Fe/H]<-1.9. The non-linearity of
the Zinn and West's
scale is significant even at 3
level. A quadratic transformation is
given to correct older values to the new scale in the range of our calibrating
clusters (
[Fe/H]
).
A minor disagreement is found at low metallicities between the metallicity
scale based on field and cluster RR Lyrae variables (via a new calibration of
the 
index) and our new cluster metallicities. It could be
tentatively ascribed to non-linearity in the [Fe/H]
relationship.
The impact of new metallicities on major astrophysical problems is exemplified
through a simple exercise on the Oosterhoff effect in the classical pair M 3
and M 15.
keywords: stars: abundances -- stars: population II -- globular clusters: general